1. Field of the Invention
The present invention pertains generally to methods and systems for obtaining geological information for application in well drilling. More particularly, the invention pertains to a method and system for continuously obtaining seismic reflection and refraction data, and utilizing the data to determine lithologic changes near the level of the drill bit, to monitor the progress of the drill bit, and to look ahead of the drill bit without significantly interfering with the drilling process.
2. Brief Description of the Prior Art
The use of sonic waves to determine the geological structure of a subterranean region is well known. In a typical application, sonic wave detectors, or seismometers, are arranged on the surface above the region to be explored. A sonic wave source, also located on or near the surface, is triggered to emit one or a series of sonic pulses. The pulses propogate in a generally spherical pattern into the ground, and are reflected at interfaces between various structures wherein the propogation characteristics of sonic waves differ. The reflected pulses travel toward the surface where they are sensed and recorded by the detectors. The resulting data are then corrected for geometric aberrations and displayed for analysis and interpretation.
This method is useful in obtaining cross-sectional mapping of the subterranean geological structure based on the sonic wave reflectances of those structural details. In the end, the accuracy and reliability of such mapping depends not only on the data gathering technique, but on the interpretive skills of the data evaluator. Usually, the mapping cannot be verified until core samples, drill cuttings, or logging data are obtained and analyzed. Although such seismic mappings are in wide use in determining where to drill for hydrocarbons, for example, information based solely on reflections of pulses generated at the surface can only be considered as of a preliminary nature for the drilling operation itself.
The method in U.S. Pat. No. 2,062,151 uses the action of the bit in either a cable tool or a rotary drilling operation to generate seismic waves which are detected by a plurality of wave detectors at the surface. The travel times of the waves to the detectors are determined and used to measure the deviation of the well hole from the vertical. Well hole deviation and the velocity of seismic waves appear to be the only types of information for which that patented method was intended. A similar method for detecting well hole deviation is shown in U.S. Pat. No. 3,817,345, wherein the seismic waves are generated by raising then dropping the drill bit.
Various well logging techniques are also well known and in widespread use to examine a well bore after it has been drilled. However, the subterranean region which can be studied by logging generally extends only a few feet from the well bore itself. (An exception to this restriction can be found in U.S. Pat. No. 3,690,164 which uses electrical or acoustical signals.) Furthermore, since logging requires a hole in which to lower the logging equipment, information from logging can be obtained only after a well bore has been drilled, and is unavailable in determining what lies ahead of the drill bit as a well is being drilled. Finally, in those cases where drilling is interrupted to log the well, the drilling string is removed, the well logged, and the drilling string replaced in the well to continue drilling. Such a round trip can require up to a day to complete, resulting in an expensive delay in drilling. As a result, when a well is logged, the operation is usually not performed more frequently than once for approximately every five thousand feet drilled.
Drill cuttings that surface with the cycling drilling mud are often examined to determine the type of material through which the drill bit has passed. But, once the well bore has been drilled to a substantial depth, it may take hours for drill cuttings to surface. By that time, the drill bit may have advanced a considerable distance beyond where the drill cuttings originated. Consequently, drill cuttings reveal nothing about the material through which the drill bit is passing when the cuttings can be examined, nor can an examination of cuttings be used to predict what lies ahead of the drill bit.
Finally, the density of a subterranean area may be determined using measurements of the weight on the drill bit, the speed of rotation of the drill bit, and the penetration rate of the drill bit, as the drill bit passes through the area in question. However, the density cannot be calculated until after the drill bit has passed through the area.